Support lookup of intermediate ID for uidmapping and gidmapping in --userns=auto

[kaivol]

In rootless mode, the host IDs in the uidmapping and gidmapping options of --userns=auto are mapped from an intermediate namespace, just like when using the --uidmap and --gidmap options.
This means that for a given host ID, the user first has to look up the intermediate ID manually.

The --uidmap and --gidmap options also support prefixing the host ID in the mapping with the @ symbol, which means that podman will look up the intermediate ID corresponding to the given host ID and it will map the found intermediate ID to the given container ID.

This PR adds the functionality mentioned above to the uidmapping and gidmapping options in --userns=auto.

`uidmapping` and `gidmapping` options of `--userns=auto` in rootless mode: 
Added the possibility to map actual host IDs (instead of intermediate IDs) by prefixing the host ID 
with the `@` symbol. 

[packit-as-a-service]

Ephemeral COPR build failed. @containers/packit-build please check.

[giuseppe]

the build is failing

[rhatdan]

@giuseppe PTAL

[giuseppe]

code LGTM, but can we please add a system test?

You can add more tests along the lines of:

diff --git a/test/system/170-run-userns.bats b/test/system/170-run-userns.bats
index 667648e5f..c4fcac3ed 100644
--- a/test/system/170-run-userns.bats
+++ b/test/system/170-run-userns.bats
@@ -147,3 +147,12 @@ EOF
     is "${output}" "$user" "Container should run as the current user"
     run_podman rmi -f $(pause_image)
 }
+
+@test "podman userns=auto with id mapping" {
+    skip_if_not_rootless
+    run_podman unshare awk '{if(NR == 2){print $2}}' /proc/self/uid_map
+    first_id=$output
+    mapping=1:@$first_id:1
+    run_podman run --rm --userns=auto:uidmapping=$mapping $IMAGE awk '{if($1 == 1){print $2}}' /proc/self/uid_map
+    assert "$output" == 1
+}

to run it, you can use the command:

$ PODMAN=bin/podman bats -f "podman userns=auto with id mapping" test/system/170-run-userns.bats

[edsantiago]

to run it, you can use the command:

$ PODMAN=bin/podman bats -f "podman userns=auto with id mapping" test/system/170-run-userns.bats

...or, even quicker/safer/better:

$ hack/bats 170:"auto with id mapping"

Better for infinite reasons, among them: it runs as root & rootless, and does a lot of invisible setup. Also obviously much more friendly.

[kaivol]

Added system test as suggested by @giuseppe.
I'm not sure if I should add more tests..?

[giuseppe]

LGTM

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: giuseppe, kaivol

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[rhatdan]

/lgtm
Thanks @kaivol

[edsantiago]

This broke CI.

[kaivol]

Just for my understanding, it was only the test that was erroneous, and that was fixed in #20979, right?

[edsantiago]

@kaivol yes, absolutely. You did nothing wrong, and the problem is (I think) now fixed. I spoke hastily this morning because I was rushing to a commitment. I apologize for leaving you in suspense like that.

Thank you again for your PR and your patience.